Carbon dioxide balance in early-successional forests after forest fires in interior Alaska

Abstract

Fire is the major disturbance in North American boreal forests, and is thought to be the most important process that determines the carbon balance in North American boreal forests. This study conducted four years of tower flux measurements in a burned ecosystem from one to four years after a fire, and nine years of measurements in a young regeneration from five to 13 years after a fire in interior Alaska. The fire scar acted as a source of 248 g C m−2 yr-1 one year after the fire, and the annual CO2 emissions continuously decreased until seven years after the fire. At the final year of the study period, 13 years after the fire, the older forest became a CO2 sink. During the 13 years after the fires, the total post-fire emissions were 767 g C m−2 across both sites. Gross primary productivity (GPP) and ecosystem respiration (RE) recovered to those of mature black spruce forests 10 years after the fire. The successional recovery of GPP was mostly explained by the recovery of the leaf area index (LAI). Anomalous weather, such as a cold spring, hot summer, and high summer rainfall, increased the CO2 emissions rather than the uptake. In interior Alaska, the post-fire CO2 emissions (35–48 Tg C) were estimated to be approximately one third to fourth of the direct CO2 emissions (156 Tg C) by combustions from 1998 to 2017, which indicates that post-fire emissions are important to the regional CO2 balance. The forest successional trajectory at young age still contains large uncertainties due to lack of data, and thus adding new data improves our understanding of the post-fire CO2 balance.